There are many systems and methods for testing the strength of a material. Generally, the strength of a material is determined by its ability to withstand an applied load. Accordingly, many testing procedures involve applying an increasing load on a material and observing when the material elastically deforms (i.e., the range in which the material does not suffer any permanent damage or distortion), when the material plastically deforms (i.e., the range in which the material still is capable of sustaining a load but permanent damage and distortion have occurred, resulting in permanent structural defects), and when the material reaches its failure point. These testing procedures are often termed ‘destructive’ because the material being tested is often destroyed or at least permanently deformed to the point that it is no longer useable for its intended application.
Additionally, conventional strength tests are generally performed ex situ and usually include applying shear, compression, and/or tensile type forces to a test material. For example, when conventionally testing a metallic or a composite component of an aircraft, the component is often removed from its position in the frame (i.e., the frame is disassembled to a certain degree) or a mock component is tested instead of the actual component and the results of the test component are deemed representative of the actual component. Continuing the example of components in aircrafts, many governmental airline agencies, such as the Federal Aviation Administration (FAA) in the United States, specify periodic maintenance checks to be performed to ensure the safe operation of all the aircrafts within their jurisdictions. For example, ‘C-checks’ and ‘D-checks’ are maintenance checks that are specified to be performed by the FAA every few years on airplanes in the United States and such checks often involve component inspection. Because conventional strength testing systems and methods are performed ex situ, these checks often involve substantial cost and expense as the testers may disassemble large portions of the aircraft. In other words, conventional testing systems and procedures are not well suited for testing the strength of components in situ.
Ultrasonic testing, however, overcomes some of these shortcomings because it is a non-destructive procedure and can be implemented in situ. Ultrasonic testing involves using transducers to impart a vibration into a material and measuring the resultant feedback vibration. Depending on the characteristics of the feedback vibration, a user can identify locations in the material where the crystal lattice has abnormalities or defects. The oscillating pressure wave imparted to the material is deemed ultrasonic when the frequency of the wave is higher than the upper limit of the human hearing range. Thus, conventional ultrasonic systems generally involve waves with a frequency in the range of between about 20 kHz (20,000 Hz) and 10 MHz (10,000,000 Hz). Comparatively lower frequency sound waves can penetrate comparatively deeper into a material than higher frequency sound waves, but the higher frequency sound waves are able to detect smaller abnormalities and defects.
However, conventional ultrasonic inspection systems are unable to detect some micro-sized and nano-sized cracks, are also unable to detect some sub-surface closed cracks, and cannot detect residual stress. This is due, in part, because the sensors and transducers conventionally used in ultrasonic inspection systems are incapable of sensing the propagation of super high-frequency acoustic vibration waves (i.e., waves with a frequency higher than about 10 MHz) through a test material. Additionally, conventional ultrasonic inspection systems are unable to adequately inspect the structural integrity of thin films and coatings. While there are sensors that are capable of detecting such high frequency vibrations, conventional ultrasound inspection systems have not used such sensors to produce 2-dimensional scans of a material. In other words, the sensors that have been conventionally used to detect such high frequency vibrations are typically large, in comparison to the cracks that they are supposed to detect, and have not been used in conjunction with other sensors. Thus, some conventional ultrasound inspection systems are used as point-by-point inspection tools and, as mentioned, have not been used to create a 2-dimensional structural analysis of a material.